You still can't force the OS to reclaim that memory, or even make Qt give it back. For example, the memory of Qt containers isn't returned to the OS but kept for subsequent (re-)allocations. STL containers behave the same, it's a (reasonable) design choice, and there's nothing you can do about it.

You're worrying about 0.4% of most users' ram. While I generally encourage responsible handling of your users' resources, I'm sure there are more important things to work on in your application than this.

Still, let me discuss my proposal:
Don't go the way to delete your UI elements, it won't work. By this you will introduce masses of bugs and instability. Apart from the already mentioned problem of non-effective memory deallocation.
Rather split your application in two: One low-resource daemon, and one GUI application. The daemon is a background process which handles the background changing. If the user wants to configure something, he starts the GUI which communicates with the daemon. If he's done, he closes the configuration program, which then really frees all that GUI-Memory. This is the only sensible approach I see for such an application.